1. Field of the Invention
This invention generally relates to a memory storage device for use in a computer system, and, in particular, to a memory device for use with a central processing unit (CPU) of a microcomputer system.
2. Description of the Prior Art
A read only memory (ROM) is well known and widely used as a memory device in a microcomputer system. The ROM is typically used to store a program and data and it may be classified into two types: (1), OR type high-speed ROM which is larger in size because of its low memory density but has a high read-out speed and (2) AND type low-speed ROM whose read-out speed is lower but whose memory density is higher, thereby allowing to obtain a large capacity while keeping compactness. Typically, given the same area, the AND type ROM can have the memory capacity twice to three times of that of the OR type ROM. However, since the read-out speed of the AND type ROM is significantly slower, the AND type ROM is not normally used by itself.
On the other hand, as shown in FIG. 1, in the case where use is made only of an OR type high-speed ROM as a ROM 4 in a one-chip microcomputer 2, as the capacity required for the ROM 4 increases, the percentage of area occupied by the ROM 4 increases significantly, so that the capacity of the ROM 4 begins to play a major role in determining the size of a chip. Under the circumstances, in an one-chip microcomputer having an integrated large capacity ROM, use is often made of an AND-OR type ROM which is a combination of the AND type and OR type ROMs. However, such an AND-OR type ROM poses difficulties when used in high-speed applications of a CPU.